1. Field of the Invention
The present invention relates generally to CO2 extraction and, more specifically, to recovery of CO2 from acidified natural seawater.
2. Description of the Prior Art
The total carbon content of the world's oceans is roughly 38,000 GtC (gigaton of carbon). Over 95% of this carbon is in the form of dissolved bicarbonate ion (HCO3−). This ion along with carbonate is responsible for buffering and maintaining the ocean's pH, which is relatively constant below the first 100 meters. This dissolved bicarbonate and carbonate is essentially bound CO2, and as shown in equation (1), the sum of these species along with gaseous CO2 represents the total carbon dioxide concentration [CO2]T of seawater, defined as:[CO2]T=[CO2(g)]+[HCO3−]+[CO32−]  (1)
At a typical ocean pH of 7.8, [CO2]T is about 2000 μmoles/kg near the surface and 2400 μmoles at depths below 300 meters. Takahashi et al., “The Alkalinity and Total Carbon Dioxide Concentration in the World Oceans,” Carbon Cycle Modeling, John Wiley and Sons, New-York, 271-286 (1981); Takahashi et al., “Carbonate Chemistry of the Surface of the Waters of the World Oceans,” Isotope Marine Chemistry, Uchida Rokakuho, Tokyo, Japan, 291-326 (1980), the entire contents of each are incorporated herein by reference. This equates to approximately 100 mg/L of [CO2]T of which 2 to 3% is CO2 (g), 1% is dissolved carbonate, and the remainder is dissolved bicarbonate. The acidification of natural seawater offers an indirect approach to recovery of CO2 from its bicarbonate and carbonate form from the equilibrium conditions of CO2 in seawater shown in equation (2). Johnson et al., “Coulometric TCO2 Analyses for Marine Studies: An Introduction,” Marine Chem., 16, 61 (1985), the entire contents of which is incorporated herein by reference. Johnson et al. demonstrated that carbonate and bicarbonate re-equilibrate to CO2 gas at seawater pH less than or equal to 6. This method has been the basis for standard quantitative ocean CO2 measurements for over 25 years.HCO3−+H+⇄H2CO3⇄H2O+CO2(g)↑  (2)
An electrochemical method to acidify seawater and recover [CO2]T simultaneously with hydrogen gas from alkaline water sources such as seawater has been developed at the Naval Research Laboratory. (U.S. patent application Ser. No. 12/958,963 filed on Nov. 17, 2011 by Felice DiMascio, the entire contents of which are incorporated herein by reference.) FIG. 1 illustrates the percent removal of [CO2]T plotted as a function of pH for effluent seawater samples taken from the electrochemical acidification cell during a 40 minute constant polarity cycle. This figure indicates that approximately 50% of the [CO2]T is spontaneously degassed from the effluent natural seawater at pH ≦4. To complement this technology, the Naval Research Laboratory has developed the present invention to enhance re-equilibrated [CO2]T (see equation 2) recovery up to 92% from seawater at pH of ≦4.